get-filename in image-snip%
Returns the name of the currently loaded, non-inlined file, or
#f if a file is not loaded or if a file was loaded with
inlining (the default).
( -> string or send an-image-snip get-filename relative-path)#f
relative-path = : boxed boolean or #f#f
The relative-path box is filled with #t if the loaded file's path is
relative to the owning editor's path, unless
relative-path is #f.